I recently investigated an attic with spray foam insulation where we observed an interesting humidity pattern. We placed data loggers near the ridge and floor of the attic as well as in the living space and outdoors.
The graph at below shows dew point data for the four locations. The really interesting part is the big difference in dew point between the highest and lowest points in the attic, shown by the red and green curves in the graph.
from Building Science http://www.greenbuildingadvisor.com/articles/dept/building-science/high-humidity-spray-foam-attics